Water-soluble polysaccharides, such as water-soluble cellulose ethers have found widespread use in many applications, such as binders, thickeners, or protective colloids. The most important properties of cellulose ethers are their solubility combined with chemical stability and non-toxicity. Water solubility and/or organo-solubility can be controlled within wide ranges by different types of ether substituents at the cellulose chain, as well as via the degree of substitution (DS) and the pattern of substitution. Accordingly, cellulose ethers are generally applied, in the dissolved or highly swollen state, to many areas of industry and domestic life, with the spectrum of applications ranging from auxiliaries in large-scale emulsions or suspension polymerization, through to additives for paints and wall paper adhesives, to viscosity enhancers in cosmetics and foodstuffs.
However, introduction of a cellulose ether which is normally in dry powder form into water or aqueous systems is frequently associated with problems, since water-soluble cellulose ethers tend to gel on the surface when added to water, and form lumps. This results in undesirably long dissolution times.
It has been known for a long time that the dissolution behavior of cellulose ethers can be influenced by means of an after- or post-treatment to counteract these problems. One known method is the reduction of the initial dissolution rate, often called “delayed or retarded solubility” to ensure that the cellulose ether is homogeneously distributed in water before starting to dissolve. This delay can be achieved, for instance, by a reversible cross-linkage with glyoxal.
U.S. Pat. Nos. 3,297,583; 3,997,508 and 5,674,999 for example, describe a process for pretreating cellulose ethers in essentially dry state with lower alkyl aldehydes and lower alkyl dialdehydes, such as glyoxal. U.S. Pat. Nos. 3,997,508 and 5,674,999 disclose that the treatment with glyoxal is followed by heating. The glyoxal treatment in U.S. Pat. No. 5,674,999 is conducted in a high intensity mixer. Unfortunately, quite high glyoxal concentrations are typically needed to achieve a sufficiently retarded solubility. Reducing the glyoxal concentrations often results in a solubility which is not sufficiently retarded for many applications. Moreover, the heating step disclosed in U.S. Pat. Nos. 3,997,508 and 5,674,999 can have a negative impact on the cellulose ether, such as partial degradation or partial discoloration, additional time needed for the heating step and the difficulty in achieving a consistent degree of cross-linking.
British Patent Specification GB 1,080,249 discloses a process wherein powdered methyl cellulose with a water content of 40 to 70% is kneaded at a pH value between 3 and 7 to a homogeneous mass with a water-soluble dialdehyde. U.S. Pat. No. 7,012,139 discloses the treatment of a moist cellulose ether having a water content of 40 to 80% with glyoxal in a twin-screw extruder in the presence of an alkali metal dihydrogen phosphate and a di(alkali metal) hydrogen phosphate. Unfortunately, the high water content has several disadvantages, such as high stickiness of the cellulose ether, which makes its handling during and after glyoxal treatment more difficult. Moreover, the high water content increases the energy demand for drying and grinding the cellulose ether.